PongSat planning

For my next PongSat I’d like to send up a system to measure and log the temperature. Maybe other quantities too, but let’s start with temperature.

Temperature decreases with altitude to about 18 km where it reaches about –60 to –70°C (http://geospatial.gsu.edu/geog1112/files/Lab3/Temperature_vs_Altitude.jpg). Above that altitude temperature increases again. PongSat flights reach altitudes of typically 100,000 feet or 30 km.

These temperatures are potentially a problem for practically everything! For instance, if you go shopping at SparkFun for temperature sensors, you find:

(and some other things that don’t look suitable.)

However, from other sources, the GE C100Y103J thermistor is rated to –80°C and is cheap and looks readily available.

Then there’s everything else. To use a thermistor you probably want to make a voltage divider with it and a fixed resistor. On a quick look it seems most carbon composition resistors are rated to –50°C. Some go to –70°C. Metal film resistors at best are rated for –65°C. (Temperature coefficients don’t look to be a problem.)

The data logger I’ve tried out, the SparkFun uLog, is built around the Atmel ATtiny24 chip and the Atmel AT45DB161D, both rated to –55°C.

We also need a battery. Here’s a plot showing the voltage of an Eveready CR2032 coin cell dropping by 20% from 21°C to –40°C, and no data below that! The Rusty Nail Workshop PongSat used a “single 1/2 AA Lithium Thionyl Chloride 3.6 volt 1.2 Ah battery”. Here it says the Saft LS 14250 operates to –60°C. I think I even have one somewhere.

Of course the only thing that must see the external temperature would be the sensor itself; the other components could in principle be insulated. But how well can you insulate the inside of a ping pong ball?

The Rusty Nail Workshop PongSat appears to have used some sort of insulation to stuff the ping pong ball. They used an Arduino Pro Mini, which is based on the Atmel ATmega328, rated to –40°C, and it seemed to work all right — though their temperature and humidity sensors bottomed out.

So maybe their insulation helped. That could be checked. I could read two sensors, one outside the insulation and one inside. Actually the ATtiny25 apparently has an onboard temperature sensor that in principle could be logged, if I wanted to reprogram it, which I don’t think I do.

Can we log something else? Barometric pressure and humidity are two obvious things. But again, finding anything rated below about –40°C is a problem. A barometric sensor should work inside insulation, though. However, I’m having trouble finding any analog barometric sensors available. There are devices like the Adafruit BMP085 Barometric Pressure/Temperature/Altitude Sensor or the SparkFun version but they’d need something like the Arduino Mini to log them… and the BMP085 only goes to 300hPa, or about 9 km altitude.

For now I’m looking at the uLog reading the GE thermistor, with the Saft battery, and some kind of insulation.

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2 thoughts on “PongSat planning

  1. Also, not that it’s critical so far as I can see, but the internal 8 MHz oscillator on the ATtiny24 is temperature dependent; it drops to about 7.7 MHz at –40°C, more or less linearly. I assume this means the uLog sampling rate will slow down proportionately.

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